Recently, as one of broadband mobile communication systems for disaster countermeasures and rescues, it is desired that video of a disaster/rescue site is wirelessly distributed to emergency vehicles (ambulance, fire engine) near the site. With respect to such video distribution systems, it is considered that the number of emergency vehicles (ambulance, fire engine) is increased depending on the scale of the disaster. If video is transmitted by unicast (individual communication) to such emergency vehicles, a shortage of wireless bandwidth may occur. Therefore, such systems need to transmit data efficiently by multicast (broadcast communication).
A conventional communication method using multicast is, for example, art described in Patent Literature 1. In the art described in Patent Literature 1, a reception terminal that returns an arrival confirmation is designated for each multicast transmission cycle. FIG. 38A is a network diagram of the art described in Patent Literature 1. In FIG. 38A, a transmission terminal 200 is a terminal which transmits a multicast. Reception terminals 201 to 203 communicate via a wireless network and receive the multicast transmitted by the transmission terminal 200.
FIG. 38B shows a configuration of a frame of each packet transmitted/received between the transmission terminal 200 and a plurality of reception terminals 201 to 203 described in Patent Literature 1. In FIG. 38B, a multicast 301 includes arrival confirmation return permission identifiers which permit the reception terminal 201 and a reception terminal 202 to return arrival confirmations, respectively, and data blocks 1 to 3 provided as a unit of error correction.
In a first cycle, the transmission terminal 200 transmits the multicast 301 to the reception terminals 201 to 203. If the reception terminal 201 detects that the received multicast 301 contains an arrival confirmation return permission identifier destined to the reception terminal 201, the reception terminal 201 transmits an arrival confirmation frame 302 to the transmission terminal 200. If the reception terminal 202 detects that the received multicast 301 contains an arrival confirmation return permission identifier destined to the reception terminal 202, the reception terminal 202 transmits an arrival confirmation 303 to the transmission terminal 200. Since the received multicast 301 does not contain an arrival confirmation return permission identifier destined to the reception terminal 203, the reception terminal 203 waits without transmitting an arrival confirmation frame.
Here, since the reception terminal 201 has successfully received the blocks 1 to 3, the reception terminal 201 returns the arrival confirmation 302 indicating that there is no error. Meanwhile, since an error occurred in block 3, the reception terminal 202 returns an arrival confirmation frame 303 for requesting retransmission of the block 3.
In a second cycle, the transmission terminal 200 retransmits the block 3, the retransmission of which was requested in the preceding cycle and additionally transmits new blocks 4 to 6, in a multicast 304. In the multicast 304, arrival confirmation return permission identifiers which permit the reception terminal 203 and the reception terminal 201 to return arrival confirmation frames, respectively, are set. Although the reception terminal 203 has successfully received the multicast 304, but did not successfully received the block 2 of the multicast 301 in the first cycle, the reception terminal 203 requests retransmission of the block 2 by means of an arrival confirmation frame 305. Meanwhile, since the reception terminal 201 successfully received the blocks 4 to 6, the reception terminal 201 returns an arrival confirmation frame 306 indicating that there is no error.
In a third cycle, the transmission terminal 200 retransmits the block 2, the retransmission of which was requested in the preceding cycle and additionally transmits new block 7 to 9, in a multicast 307 in the same manner as in the second cycle. In the multicast 307, arrival confirmation return permission identifiers which permit the reception terminal 202 and the reception terminal 203 to return arrival confirmation frames, respectively, are set. Since the reception terminal 202 has successfully received both of the multicast 304 and the multicast 307, the reception terminal 202 returns an arrival confirmation frame 308 indicating that there is no error. Meanwhile, since the reception terminal 203 did not successfully received the block 8 of the multicast 307, the reception terminal 203 returns an arrival confirmation frame 309 for requesting retransmission of the block 8.
As described above, in Patent Literature 1, the transmission terminal 200 adds to a multicast the information of identifiers of reception terminals that the transmission terminal 200 permits to return arrival confirmation frames, thereby designating the reception terminals that return arrival confirmation frames. This has enabled prevention of collision between arrival confirmation frames, and reduction of time necessary for reception processing of the arrival confirmation frames at the transmission terminal, and thus realized improvement of the communication quality.
Another conventional communication method is, for example, art described in Patent Literature 2. In the art described in Patent Literature 2, destination information of a reception terminal (arrival confirmation return request) that has successfully received a multicast is removed from the multicast to be retransmitted, and the resultant multicast is retransmitted. FIG. 39 shows an example of a data transmission/reception sequence of the art described in Patent Literature 2. The communication system of the art described in Patent Literature 2 includes a transmission terminal 400 which transmits a multicast, and reception terminals 401 to 405 which each receive the multicast and transmit an Ack or a NAK as a response.
In FIG. 39, first, the transmission terminal 400 transmits a multicast 500 which includes destination information of a reception terminal 401 to a reception terminal 405. Since the reception terminals 402 and 405 have successfully received the multicast 500, the reception terminals 402 and 405 return Acks 502 and 505, respectively, each indicating that there is no error, to the transmission terminal 400. Meanwhile, since the reception terminals 401, 403, and 404 have not successfully received the multicast 500, the reception terminals 401, 403, and 404 return NAKs 501, 503, and 504, respectively, for requesting retransmission of the multicast 500.
Next, since the transmission terminal 400 has received the Acks from the reception terminals 402 and 405, the transmission terminal 400 transmits a retransmission multicast 500, which is the multicast 500 from which destinations of the reception terminals 402 and 405 has been removed. Since the reception terminal 403 has successfully received the retransmission multicast 500, the reception terminal 403 returns an Ack 513 indicating that there is no error. Meanwhile, the reception terminals 401 and 404 have not successfully received the retransmission multicast 500, the reception terminals 401 and 404 return NAKs 511 and 514 for requesting retransmission of the retransmission multicast 500.
Next, since the transmission terminal 400 has received the Ack from the reception terminal 403, the transmission terminal 400 transmits a retransmission multicast 500, which is the retransmission multicast 500 from which the destination of the reception terminal 403 has been removed. Since the reception terminal 404 has received the retransmission multicast 500 successfully, the reception terminal 404 returns an Ack 524 indicating that there is no error. Meanwhile, since the reception terminal 401 has not successfully received the retransmission multicast 500, the reception terminal 401 returns a NAK 521 for requesting retransmission of the retransmission multicast 500.
As described above, in the art described in Patent Literature 2, the destination information of the reception terminal that has successfully received a multicast is removed from the multicast and the resultant multicast is retransmitted, such that the reception terminal that has already successfully received the multicast does not return an Ack. Accordingly, the number of returns of unnecessary Acks has been reduced and the possibility of collision between the Acks (NAKs) has been reduced. In addition, the time necessary for reception processing of Acks (NAKs) at the transmission terminal has been shortened, and the communication quality has been improved. Further, in the art described in Patent Literature 2, a priority order is assigned to the arrival confirmation frame to be returned, in accordance with the order position set to each piece of destination information included in a multicast. Then, a reception terminal having the highest priority returns an Ack immediately, and the other reception terminals randomly determine the order of return Acks in accordance with the possibility determined by using their own priority orders. Accordingly, control in accordance with the priority of the reception terminals has been realized.